function [F] = coefficient(x,v2,p,W_b4)

        W_a4 = x(2);
        K_a1 = p(9);
        K_a2 = p(10);
        K_w =  p(11);
        H_plus = v2;

        F = (K_a1/H_plus + (2*K_a1*K_a2)/H_plus^2)/(1+K_a1/H_plus+K_a1*K_a2/H_plus^2);
        
    end